1. Field of the Invention
This invention relates to a method and apparatus for combustion of an oxygen/liquid fuel mixture in a manner which produces a fishtail or fan-shaped flame. Fishtail or fan-shaped flames in high temperature industrial furnaces, such as glass melters, provide enhanced flame coverage resulting in more uniform heat distribution and relatively high radiative heat transmission compared to conventional liquid fuel burners.
2. Description of Prior Art
A frequently encountered issue with 100% oxy-fuel fired, high temperature industrial furnaces, such as glass melters, is the requirement for enhanced flame coverage, particularly in applications involving liquid fuel-oxygen burners. Many conventional burners use a cylindrical burner geometry in which the liquid fuel and oxidant are discharged from a generally cylindrical nozzle, producing a flame profile which diverges at an included angle of 20.degree. to 25.degree., in a generally conical shape. Conventional oxy-fuel burners, such as are taught by U.S. Pat. No. 5,199,866, having a cylindrical or cone-shaped flame configuration can create hot spots in such furnaces due to limited flame coverage, that is, projected flame area over the load surface, and concentrated heat release through a relatively small flame envelope. Such hot spots, in turn, lead to furnace refractory damage, primarily on furnace crown and/or opposite side walls, increased batch volatilization, and uncontrolled emissions of NO.sub.x, SO.sub.x, and process particulates. Previous efforts at controlling liquid fuel flame characteristics, in particular flame length, have focused on variations in mean liquid fuel droplet size (denoted by MMD, Mass Median Diameter). See, for example, U.S. Pat. No. 5,251,823 which teaches an adjustable atomizing orifice liquid fuel burner. Some of the more common means for varying liquid droplet size include varying liquid fuel velocity (by varying flow orifice area), atomizing media velocity (by varying flow orifice area), mass flow rate of atomizing media, type of atomizing media (air, oxygen, steam, etc.), and changes in nozzle design (introducing swirl, internal mixing, etc.). However, the extents to which flame lengths can be increased are generally limited due to the size of the particular industrial furnace.
Other known techniques for increasing flame coverage include staggered firing in which a number of oxy-fuel burners on each side of a furnace are staggered in placement to improve the effective load coverage through the use of conical expansion of individual flames. Such an arrangement generally provides better heat distribution than opposed-fired burners. However, such a staggered arrangement can create undesirable cold regions in pocket areas between adjacent burners. To resolve this problem, the number of burners used by a given furnace can be optimized to effectively utilize the load space; however, increasing the number of burners also leads to higher capital costs in burner and flow control equipment and, thus, is not necessarily desirable.
U.S. Pat. No. 5,217,363 teaches an air-cooled oxygen gas burner having a body which forms three concentric metal tubes supported in a cylindrical housing and positioned about a conical bore and a refractory side wall of a furnace. The three concentric tubes can be adjusted with respect to each other to define a nozzle with annular openings of variable size for varying the shape of a flame produced by a mixture of fuel, oxygen and air. The air is fed through an outer chamber for cooling the concentric tube assembly and the furnace refractory positioned about the burner nozzle.
U.S. Pat. Nos. 5,256,058 and 5,346,390 disclose a method and apparatus for generating an oxy-fuel flame. The oxy-fuel flame is produced in a concentric orifice burner and, thus, results in a generally cylindrical flame. A fuel-rich flame is shielded within a fuel-lean or oxygen-rich flame. The flame shielding is controlled in order to achieve a 2-phase turbulent diffusion flame in a precombuster, in order to prevent aspiration of corrosive species, and also to reduce nitrogen oxides formation.
U.S. Pat. No. 5,076,779 discloses a combustion burner operating with segregated combustion zones. Separate oxidant mixing zones and fuel reaction zones are established in a combustion zone in order to dilute oxidant and also to combust fuel under conditions which reduce nitrogen oxides formation.
It is apparent that there is a need for an oxy-fuel burner, in particular, an oxy-liquid fuel burner, which can be used in high-temperature furnaces, such as glass melting furnaces, which provides uniform heat distribution, reduced undesirable emissions, such as nitrogen oxides and sulfur oxides, and which produces a highly radiative and luminous flame.